As is stated in the Brown et al patent, in the operation of nuclear reactors such as pressurized water reactors (PWR's) or in boiling water reactors (BWR'S), it has been found that debris such as nuts, bolts, metal cuttings, wires, and drill bits sometimes accumulate in the reactor during construction, repair or the like. Certain mid-range sizes (1/2" to 4") of this type of debris are particularly troublesome, since that debris is likely to be carried by cooling water to the area near the bottom (lower ends) of the fuel rods. The debris vibrates in the moving coolant and impacts principally upon lower ends of the rods, ultimately abrading and causing fretting wear of the fuel rod cladding at that point. This type of wear is recognized as a significant cause of fuel failures. As is noted in the Brown et al patent, one prior approach to this problem was to use extra long solid lower end caps on the fuel rods. The end caps did not contain fuel and therefore there would be no escape of radiation if extensive fretting wear occurred in the end caps. However, that approach of using elongated end caps reduces the fuel column length and may result in a reduction of power output for a given overall size of the reactor.
In the Brown et al. patent, a lowermost grid spacer is described which is positioned on or only slightly above the lower tie plate. The geometry of the Brown grid spacer is arranged to divide coolant flow openings in the lower tie plate into smaller openings and thereby trap at least part of the debris in the zone near the lower tie plate before the debris comes in contact with the fueled portion of the rods.
As an indication of the significance of the debris problem, reference may be made, for example, to recently issued U.S. Pat. such as Nos. 4,652,425--Ferrari et al , granted Mar. 24, 1987; No. 4,684,495--Wilson et al., granted Aug. 4, 1987; No. 4,684,496--Wilson et al., granted Aug. 4, 1987; No. 4,781,884--Anthony, granted Nov. 1, 1988; No. 4,828,791--De Mario, granted May 9, 1989 and No. 4,832,905--Bryan et al , granted May 23, 1989.
While a number of the foregoing proposals to reduce the debris problem have focused remedial attention on the region in the vicinity of the lower tie plate, certain of the proposals have required that additional space in the vicinity of the tie plate be taken from the length of the active fuel rods in order to insert means to accomplish the desired collection of debris. Other proposed approaches also may result in an unacceptable drop in coolant pressure, thereby adversely affecting the desired heat transfer to the coolant.
The present invention, on the other hand, is directed towards a lower tie plate assembly incorporating an integral, improved debris screen or alternatively, having an added debris screen positioned within the confines of an existing lower tie plate but which is capable of trapping significant additional debris.